1. Field
Embodiments described herein generally relate to adhesive compositions having a reduced cure time and methods for making and using same. More particularly, such embodiments relate to adhesive compositions having a reduced cure time that include one or more phenolic-aldehyde resins and one or more cure accelerants and methods for making and using same.
2. Description of the Related Art
The preparation of phenolic-aldehyde adhesive compositions, e.g., phenol-resorcinol-formaldehyde (PRF) resins, typically involve forming a solution of a water-soluble, fusible phenolic-aldehyde resin and making an adhesive mix therefrom by adding to the resin solution a methylene donor that can act to cure the resin to a cross-linked state. Common methylene donors or “hardeners” include, for example, aqueous formaldehyde, paraformaldehyde, hexamethylenetetramine, and the like.
One drawback with phenolic-aldehyde adhesives is that these adhesives tend to be slow to cure at ambient or “cold” pressing conditions. A slow cure rate equates to a slow gel time, which translates into slow product production and/or, depending on the particular product, difficulty in making a product with the phenolic-aldehyde adhesives. Slow gel times mean that the phenolic-aldehyde adhesives remain fluid, i.e., have a low viscosity, which it makes it difficult, if not impossible, to evenly apply the adhesive to wood or other substrate surfaces, especially vertically oriented surfaces.
Typically, the cure rate for a particular adhesive mix is often adjusted with an alkali metal hydroxide and/or alkali metal sulfite cure accelerants. For example, alkali metal hydroxides such as sodium hydroxide (NaOH) and/or potassium hydroxide (KOH) are commonly used to speed the cure rate, i.e., reduce the gel time, of phenolic-aldehyde adhesives in wood laminating processes. These accelerants, however, reduce the viscosity of the adhesive. Reducing the viscosity of the adhesive leads to lower quality composite products and/or composite products having inconsistent strength. These accelerants are also incompatible with Radio-Frequency (RF) presses because the metals present tend to cause arcing in the press. Additionally, the degree the cure time can be accelerated with such catalysts or accelerants is limited because there is a point where the metal hydroxides solubilize the adhesive and can cause the composite products to swell during curing of the adhesive.
There is a need, therefore, for improved adhesive compositions having increased viscosity, reduced metal concentration, and/or a reduced cure time for making composite products.